Arrangement for electrically charging a beam of microparticles with an ion beam



March 25, 1969 D. O. HANSEN ET AL ARRANGEMENT FOR ELECTRICALLY CHARGING A BEAM MICROPARTICLES WITH AN ION BEAM Filed Dec. 5, 1966 @SEEE 222cm AGEN T.

United States Patent O 3,435,208 ARRANGEMENT FOR ELECTRICALLY CHARGING Egll/M OF MICROPARTICLES WITH AN ION David O. Hansen, Westminster, Joseph F. Friichtenicht, San Pedro, and Neal L. Roy, Redondo Beach, Calif., assignors to TRW Inc., Redondo Beach, Calif., a corporation of Ohio Filed Dec. 5, 1966, Ser. No. 599,242 Int. Cl. H01j 39/34; B01d 59/44 U.S. 'Cl. Z50-41.9 6 Claims This invention relates to means for electrically charging particles of microscopic size, and more particularly to means whereby such charging is accomplished by subjecting the particles to the influence of a high intensity ion beam.

Electrostatically charged microscopic particles find use in the laboratory for simulating micrometeoroids. One type of apparatus for electrostatically charging microscopic particles by means of an ion beam is disclosed by J. F. Vedder in an article entitled, Charging and Acceleration of Microparticles, published in Review of Scientiiic Instruments, vol. 34, No. 11, pp. 1175-1183, dated 1963. In that lapparatus the particle to be charged is trapped in an electrodynamic containment system and bombarded by a low intensity ion beam. The parameters of the containment system `are adjusted to increase the charge on the particle. When the particle is fully charged, it is expelled from the charging chamber. The main disadvantage of the foregoing method is the extremely low charging rate; it is estimated that only four particles :a day can be charged in that manner.

The present invention provides apparatus whereby a line beam of microparticles is injected into a high intensity ion beam substantially coaxially therewith. The ion beam induces a surface field on the microparticles whose magnitude is proportional to the time during which the particles remain within the ion beam. Thereafter, the ion beam is separated from the charged particle beam.

In the drawing, FIG. 1 is a diagrammatic view of lapparatus for charging microparticles by means of an ion lbeam according to the invention; and

FIG. 2 is a schematic of a variable D.C. supply used in the apparatus of FIG. 1.

Referring to FIG. 1 of the drawing, there is shown a vacuum chamber containing a particle injector 12 for accelerating micronsize particles along a first axis 14 in f the direction indicated by the arrow 15. The particles may be made of iron, carbon or aluminum, for example, and may range in size from fractions of a micron to a few microns in diameter. The particles leaving the particle injector 12 may be initially charged to electric field strengths of the order of 108 volts per meter and have a velocity up to a few hundred meters per second. However, as will be apparent, the initial field strength is not` critical, because of the relatively large final iield strength that will result from interaction with the ion beam. However, at least a nominal field strength is necessary to focus and align the particles relative to the ion beam. The particle injector 12 may comprise a particle accelerator of the kind disclosed in the article by J. F. Friichtenicht, published in the Review of Scientific Instruments, vol. 3, No. 2, pp. 209-212, February 1962, and entitled, Two-Million-Volt Electrostatic Accelerator for Hypervelocity Research, or in the article by H. Shelton, C. D. Hendricks, Jr., and R. F. Wuerker, published in the Journal of Applied Physics, vol. 31, No. 7, pp. 1243, July 1960, and entitled, Electrostatic Acceleration of Microparticles to Hypervelocities. A suitable particle injection rate is about 10 particles per second.

The particles leaving the particle injector 12 are focussed by a particle beam lens 16 into a line particle 3,435,208 Patented Mar. 25, 1969 beam 18, indicated by the arrows, having a diameter, for example, of about 1 millimeter. The lens 16 is of conventional type well known to those skilled in the art of electron optics.

A pulsed ion source 20 generates a high intensity stream of ions along a second axis 22 in the direction indicated by the arrow 23. The ions are focussed by an ion beam lens 24 into a fine ion beam 26, indicated by the arrows. The ion source 20 is of conventional type, and may be one of the kind disclosed in the articles entitled, Production of Low Divergence Positive Ion `Beams of High Intensity, by N. B. Brooks, T. H. Rose, A. B. Wittkower, and R. P. Bastide, Review of Scientific Instruments, vol. 35, No. 7, pp. 894-901, July 1964. The ion beam lens 24 is also of conventional type known to those skilled in the art of electron optics. The ion beam 24 may, for example, have an energy of 50,000 electron volts, a current of microamperes and a beam diameter of about eight millimeters.

Spaced downstream from the particle beam lens 16 and ion beam lens 24, are a pair of spaced 4apart particle beam deflecting plates 28 and 30 laying on opposite sides of the ion beam axis 22. The plates 28 and 30 `are adapted to have a direct current voltage applied to them by a variable D.C. supply 31 that may be varied from about 1 to 2 kilovolts. When the particle beam 18 is directed between the plates 28 and 30, the electric field therein acts to deect the particle beam 18 so that it coincides with the ion beam axis 22. By Varying the voltage on the plates 28 and 30, the particle beam 18 may be deflected as to lie parallel with the ion beam axis 22. By moving the plates 28 and 30 longitudinally in either direction parallel to the ion beam axis 22, the axis of the particle beam 18 may be made to coincide with the ion beam axis 22.

A particle detector 32 is disposed in the path of the ion beam 26 and particle beam 18. The particle detector 32 may be one of the kind disclosed in the above-identified Shelton et al. article. When a particle passes through the particle detector 32, the latter generates a rectangular voltage pulse whose amplitude is proportional to the charge on the particle and whose time duration is equal to the time of liight of the particle through the detector 32.

The rectangular voltage pulse from the particle detector 32 is fed to a discrimnator 34 which differentiates the rectangular voltage pulse and generates a trigger pulse coincident with the trailing edge of the rectangular pulse. The discriminator 34 may be one of the kind disclosed by C. B. Ward and C. M. York in Nuclear Instruments and Methods, vol. 23 (1963), pp. 213-217.

The trigger pulse from the discriminator 34 is fed simultaneously to first and second one shot multivibrators 36 and 38 that generate positive rectangular pulses of equal time duration. The multivibrators 36 and 38 may be of the -kind disclosed in the textbook yby J. Millman and H. Taub, entilted, Pulse, Digital, and Switching Waveforms, chapter l1, published by McGraw-Hill, New York, 1964.

The tfirst multivibrator 36 'drives a high voltage pulse generator 40 whose output is fed to the extraction electrode of the ion source 20, thereby tur-ning on the ion beam 26 for the period of the first multivibrator 36. The high voltage pulse generator 40 may be one of the kind disclosed in the article entitled, Transistor Circuit Pulses 1000 Volts, by D. O. Hansen, Electronics, vol. 38, No. 18, dated 1965.

When the ion beam 26 is turned on, it overtakes the particle that just exited the particle detector 32 in a chargin'g region identified by broken lines 56. In t-he charging region 56, the particle is subjected to the charging influence of the ion beam 26. While it is within the ion beam 26, t-he surface eld strength of the particle is increased at a constant rate, regardless of the size of the particle. The nal field strength reached on the particle is determined by the time that it stays Within the beam. The ion beamzi suibjects the particle to an electricdiel'd which tends to displace the particle radially, and when the particle moves out of the ion beam 26` the charging process ceases. Thus, the closer to the ion beam axis 22 the particle is injected, the longer will be the charging time before the particle is deflected out of lche ion beam 26. f

The time required for the particle A to reacha speciiied surface eld is inversely proportional to the ion beam current density. The maximum obtainablefparticle" surface iield strength varies as the third rootnof the ion beam velocity and also'las the third root .of the particle'lvdiameter. An ion beam of Q microampereslcurrent(andAjbeam diameter of 8` millimeters/will;increase'jthe lsurface field strength of an'ironV particle olf 2 m'crfons'gdianietr by-flOfk to 109 volts'per'fmetlerf While* aflv` amperefionbeam will increase, the 'el-d strength' ofI the same particle by up 'to 4 1010' volts per'meter '-1 1 fl ,i Aftertheparticle'is chargedtheion bearn26 Iis"tilrneiltl oty and ,the voltage ,is

l l 4reappiin fqfth paniek, fd'eflting plates 28 and 3Q 'sothat the next particlel will'bededected onto the ion beamfaxis'ZZ.' Thefch'arging lperiod Iis 'deteri-V mined bythel duration 'the multivibrator pulses.f Wl|1e,n

Athe multivibrator pulses terminate, the ionv beam 2Q autqmaticallyturnsbir and the particle beam deflection voltageis restored. z .'vl

The charged particle and `the ion beam 2d pass through a D.C. magnetic riield region, which maybei'provitied by a permanent magnet 58. The D.C. rriag'netic'dield, |whicl1 has a strength of 5 to 10 kilogaussghas a direction 'perpendicular to the ion beamvaxis 22," as illustrated vbyarroW' vheads 60 emanating from the plane of the"drawing',.f[[`he magnetic iielid 60 detiectsl the ion beam 26 ,into la Faraday 4 cup r62. The Faraday cup 62 may include an electrometer for measuring the ionbeam current, from which thecurrent density ofthe ion gbeam 26 may be calculated.fBecause of the lower charge to mass ratio and lower`,velocity,of the charged particle, the magnetic eld has', no eiect on the charged particle, and the particle Ibeam lland ion beam 26 are thereby separated from one another.,Tli'e Iparticle beam 18 may then be fed to a utilization device,

not shown.

, The embodiments of the invention in which an exclusive property, or privilege is claimed are defined as follows:

1. Particle charging apparatus, comprising: means for producing and accelerating a beam of microparticles along an axis; means for producing an ion beam that intercepts said microparticles in the same -direction along said axis so as to impart ysurface charge to said microparticles;

g nd meariswiorA separating said ion beam from said charged microparticles.

2. rIlhe invention according to claim 1, and further including means for detecting particles traversing a iirst region;

and means responsive to said detecting means for causing Asaidion` beam to intercept said particles in a second region beyond said first region.

3. The invention according to claim 2, and further including means forming a region of magnetic eld beyond said second-'region and traversed by 4said particle and ion beams fondeecting said ion beam'from the path of charged particles.

4. Particle charging apparathus, comprising: means for producing and accelerating a beam of microparticles directed alongarst axis; i means for producingan ion beam directed along'a secf Mond axis;` .Y i l f lmeans for deflecting said microparticle beam into coincidence with 4said ion beam along said second axis andin'the sa'me dire'ctio'n thereof,N whereby microparticles within said'ion beam receive surface charge and'means' for separating said ion beam from said charged microparticles.

5. The invention according to claim 4, and further including meansiri the path of said deflected microparticle `beam fo'sensinglthe deflected microparticles and for energizing said' ionlbeam producing means to cause the ion lbeam to-interceptV said deflected microparticles after they traverse saidmicroparticle sensing means.

sponsive tosaid microparticle sensing 'means for del-energizing said microparticle deflecting means during traversal by saidnion beam.

References Cited .UNITED STATESPATENTS 2,772,363 11/1956 Robinson 25o- 41.9 3,300,640 1/1967 .nubankv 25o-49.5

RALPH G. NILSON, Primary Examiner. yA. L. BURCH, Assistant Examiner.

Y U.S. Cl. XR. 

1. PARTICLE CHARGING APPARATUS, COMPRISING: MEANS FOR PRODUCING AND ACCELERATING A BEAM OF MICROPARTICLES ALONG AN AXIS; MEANS FOR PRODUCING AN ION BEAM THAT INTERCEPTS SAID MICROPARTICLES IN THE SAME DIRECTION ALONG SAID AXIS SO AS TO IMPACT SURFACE CHARGE TO SAID MICROPARTICLES; 